The present invention relates to a developing unit included in an electrophotographic copier, facsimile apparatus, printer or similar image forming apparatus and, more particularly, to a developing unit using a developer consisting of toner and carrier and capable of separating the toner and carrier before collecting the developer, and a method of collecting the developer from the developing unit.
It is a common practice with an image forming apparatus to electrostatically form a latent image representing a document image on the surface of a photoconductive element, or image carrier, develop the latent image by a developer stored in a developing unit, transfer the resulting toner image to a paper, and then fix the toner image on the paper. The developing unit is selectively operable with a monocomponent developer or a bicomponent developer, as customary in the image forming art. The monocomponent developer is charged toner and directly deposited on the surface of the image carrier. The bicomponent developer is a mixture of toner and carrier and has the toner frictionally charged and deposited on the carrier, so that the carrier carrying the toner therewith is supplied to the surface of the image carrier. In both the monocomponent and the bicomponent developers, the toner is repeatedly supplied to the image carrier until it has been deposited on the toner image. Specifically, after the development of the latent image and the following image transfer, the toner remaining on the image carrier is collected by a collecting device for recirculation. Specific constructions of the toner collecting device are disclosed in Japanese Patent Laid-Open Publication No. 57-19774 and Japanese Utility Model Laid-Open Publication No. 4-114066.
Regarding the bicomponent developer, the carrier deteriorates due to aging and, in many cases, causes defective charging to occur. When defecting charging occurs, the toner becomes excessively great in amount and flies about to invite defective images. In such a case, the developer stored in the developing unit is replaced with a fresh developer by a serviceman. Specifically, a serviceman opens, for example, a shutter mounted on the bottom of the casing of the developing unit, removes the used developer, introduces a fresh developer, and the closes the shutter. This, however, brings about a problem that since the whole developer is replaced, even the toner which is still usable is simply discarded. For example, assume that the toner concentration of the developer is 3 wt %, and that 1 kg of developer is stored in the developing unit. Then, the developer contains as much as 30 g of toner. When a document of size A4 and having an image area ratio of 6%, about 150 copies are available with 30 g of toner.